The greatest hesitation I have about the market economy is the wedge between demand and the willingness to pay that is the ability to pay. This is, after all, why full employment is so important. This is why opportunity and equality are so important.... This wedge... presents] insurmountable problems... [to]any kind of welfare claims about the market economy.... We can't say anything about the quality of the system as a whole that isn't contingent on implausible assumption about the quality of the distribution of the ability to pay. The trouble is the ability to pay is tied up with productivity, and this doesn't seem to be a justifiable basis for distributing well-being. At the same time allocating productivity efficiently is the only chance of getting a surplus in the first place to distribute.
If we have robots, robot socialism is probably an answer. Until then I just don't know.